Process miniaturization and micro-reactor technology provide opportunities for improving chemical processing and provide improved process control in chemical synthesis. Smaller scale chemical reactors may allow safer and more efficient investigation of the kinetics of chemical reactions. Recent work has investigated the production of chemical processing equipment on silicon wafers using microfabrication techniques. Microfabrication techniques involving photolithography, deep-reactive-ion etching, thin-film growth and deposition, and multiple wafer bonding provide the opportunity to design and fabricate novel chemical reactors, and certain simple chemical reactor systems have been produced incorporating various unit operations.
Chemical synthesis may be optimized by precise control of the parameters that determine the overall efficiency of the reaction. These parameters include, for example, reaction temperature, pressure, degree of mixing of reactants, rate of diffusion of the reactants to a catalytic site, and formation of byproducts by inadvertent exposure to products or other unintended compounds. Certain chemical reactions occur rapidly and, therefore, are particularly difficult to control with precision. Inadequate mixing may lead to regions of the reaction mixture including non-stoichiometric portions of reactants, reaction hot spots, reaction dead spots due to channeling, formation of unwanted byproducts, or production of products other than those desired.
The above problems may be particularly evident when reactions are carried out on a large scale, with relatively large volumes of reaction components. Typical industrial chemical processing equipment is designed to hold a relatively large volume of materials. The large size of these reactors makes controlling high rate chemical reactions especially difficult. Proper mixing of the components of the reaction is necessary to optimize reaction rates and overall reaction yields. Incomplete and inefficient mixing may also affect controlling the temperature of a reaction. Localized hot spots due to a high concentration of reactants typically lead to product degradation and by-product formation. In the case of a conventional tank reactor, for example, when reactants are introduced they are typically introduced into the reactor in separate inlet ports and then mixed together by agitation of the entire reactor contents. For chemical reactions that occur rapidly, the reaction will likely have occurred before adequate mixing has been accomplished. Under these conditions, undesired secondary reactions may occur which produce undesired byproducts, reduce the overall efficiency of the reaction, and create downstream purification problems.
The limitations of conventional industrial and laboratory reactor systems may be primarily attributed to the small surface to volume ratio present in the reactors. This ratio greatly reduces mixing efficiency, and uniform temperature control of the entire volume of chemical reactants is difficult. In recent years, an effort has been made to overcome these limitations. Micro-reactors have been fabricated using known methods of microfabrication developed in the computer chip manufacturing industry. Methods of microfabrication available for the production of micro-reactors typically include, but are not limited to, photolithographic and etching techniques. Such techniques typically are utilized in the formation of configurations on silicon wafers on which silicon dioxide, silicon nitride and similar thin films have been applied. Using these techniques, three-dimensional micro-machined devices may be produced having dimensions from one to over a hundred microns with submicron tolerances. Micro-machining methods have been used to produce miniature chemical reactors having significantly higher surface to volume ratios than their macro-scale counterparts. Although the high surface to volume ratios of miniature chemical reactors offer improvements in thermal management and mass transfer, designing reactors of this type also presents challenges resulting from greatly increased pressure drop across channels having dimensions that may be on the order of tens of microns. These pressure drop considerations result in increased energy costs and limitations on the overall production rate of the reactor.
In general, the performance of chemical synthesis is controlled by mass transport and thermal transport through the reaction medium. Especially in heterogeneous catalytic reactions, which include many industrially important chemical reactions, the diffusion rate of the reactants into the liquid, through the liquid, and onto the catalytic reaction site control the rate of reaction. As used herein, a heterogeneous catalytic reaction is one wherein the reaction of gaseous and/or liquid reactants is promoted by the presence of a solid catalyst.
Multi-phase industrial reactors operated continuously are usually classified according to their flow dynamics. As an example, in trickle bed reactors the free volume of the reactor not occupied by catalyst is predominantly the gas phase and the liquid phase forms a thin film on the catalyst pellet. Uneven flows can lead to incomplete utilization or local zones of varying reaction rate and heat transfer. Poor distribution of the fluids can thus lead to local “hot spots” which can decrease the reaction selectivity, reduce catalyst life, or lead to side reactions which may lead to dangerous uncontrollable side reactions. For example, hydrogenation reactions are exothermic and consequently thermal uniformity and adequate temperature control is a primary concern. In addition to losses in selectivity and decreased product yield, poor thermal management can decrease the life of a catalyst and further reduce the efficiency of a reactor. The associated flammability danger of oxidation reactions precludes the use of organic solvents or high concentrations of oxygen. In a micro-reactor, however, where the area of elevated temperatures due to exothermic reactions and reaction zone itself is confined to a small volume, more aggressive reaction conditions can be tolerated.
Micro-reactors may be produced having extremely low surface to volume ratios and such reactors, therefore, provide unique opportunities for reaction and chemical engineers to control transport phenomena to produce extremely efficient chemical reactors. In addition, by constructing reactors using silicon micro-machining methods, the opportunity to integrate a wide variety of sensing elements may allow more efficient control and rapid probing of chemical kinetics.
Considering the above-described advantages of microfabricated chemical reactors, i.e., micro-reactors, it would be advantageous to provide an improved design for such a micro-reactor, and particularly for an improved micro-reactor adapted to perform heterogeneous catalytic reactions.
It also would be advantageous to provide a micro-reactor having a low surface to volume ratio and increased thermal and mass transfer, and which exhibits a relatively low pressure drop across the reaction channel under reaction conditions.
In addition, it would be advantageous to provide a micro-reactor that is designed to more rapidly and efficiently mix the reaction components of a heterogeneous reaction.